[Encyclios]

Quasi-static transformation

A quasi-static transformation is defined as a thermodynamic transformation that occurs extremely slowly, so that the thermodynamic system under examination, passing from an initial equilibrium state A to a final equilibrium state B, goes through a succession of infinite equilibrium states, separated by infinitesimal transformations and infinitesimal variations of the system properties. If we want to identify a finite transformation, it is necessary to know, in addition to the initial and final states, all the infinite intermediate states through which the system passes and then all the values that define each state.

Let’s consider a system in thermodynamic equilibrium; if we modify, by an infinitesimal amount, some of the properties of the environment in order to alter the equilibrium between the environment and the system, the system will undergo an infinitesimal transformation that will bring it in a new equilibrium condition. Then, if we realize a finite transformation through a succession of infinitesimal transformations, we obtain a so-called quasi-static transformation: it is therefore characterized by the fact that, in every instant, the system is, unless infinitesimal, in a condition of thermodynamic equilibrium.

Let’s consider a closed system cylinder-piston containing fluid in equilibrium conditions at a certain pressure and temperature; we want to double the fluid pressure while maintaining unchanged the temperature: we can do this by putting the system in contact with a source that is at the same temperature and applying instantly on the piston an adequate weight. In this way, the system is brought to the desired final equilibrium conditions, but through a transformation during which it is never in equilibrium. The transformation is therefore not almost static. We could however proceed in another way: always putting the system in contact with the source that keeps it at constant temperature, we can successively increase the weight applied on the piston of an infinitesimal quantity and wait, at each increase, the reaching of the equilibrium. In this way, the transformation is constituted by a succession of equilibrium states and is therefore almost static: in particular, it is an isothermal transformation. It was not isothermal the previous transformation: in fact, in that case, during the transformation it was not possible to define the thermodynamic state, so it was not possible to talk about internal properties and, in particular, about temperature.